1. Field of the Invention
The present invention relates to an elastic shaft coupling which is used in a steering apparatus, or the like, of a car, and a method of manufacturing an element of the coupling and, more particularly, to a technology for reducing the weight or the manufacturing cost of the coupling while maintaining a sufficient strength of a stopper portion.
2. Related Background Art
A steering apparatus of a car, or the like, is comprised of a steering wheel to be used for steering operation of the driver, a steering gear for steering the wheels of the car, and a steering shaft used for coupling the steering wheel and the steering gear. Then, in the steering apparatus of a car, the steering gear is seldom positioned on the center axial line of the steering wheel, so that a plurality of steering shafts which are coupled to each other by universal joints are often used. As the universal joint for the steering shafts, a Cardan joint having a cross piece (cross shaft) which is inserted between a pair of coupling elements to be rockable is generally used, as disclosed in the U.S. Pat. No. 3,501,928.
Recently, in order to reduce transmission of kickback, or the like, from a road surface to the steering wheel, an elastic shaft coupling which employs an elastic member such as synthetic rubber is proposed, as in the Japanese Patent Application Laid-Open No. 10-89373. In an elastic shaft coupling of this type, one of the coupling elements of the Cardan joint is divided into a joint member (i.e., a yoke) and a shaft, and an elastic ring formed of synthetic rubber, or the like, is inserted between the yoke and the shaft. In order to prevent damage, or to improve the durability of the elastic ring, a stopper portion for restricting a relative rotation within a predetermined amount is formed between the yoke and the shaft.
Incidentally, in the elastic shaft coupling described above, if a so-called stopper strike is repeatedly brought about due to a load of a large torque, it is required to prevent deformation or damage of the joint member or the stopper portion of the shaft, which does not become a significant problem since the yoke is normally formed by pressing or forging from a steel plate, or the like, having a comparatively large thickness. However, the size of the shaft is inevitably reduced since the shaft is normally set inside a yoke member. As a result, it becomes difficult to maintain the strength of the stopper portion.
For this reason, though the shaft may be formed by a multiple stage forming machine from a carbon steel bar having a comparatively high hardness, there arises a problem in this case that the manufacturing cost of the shaft is increased due to a large size of the forming machine or an increased number of forming molds. For instance, in order to form an elongated shaft by a forming machine, a comparatively large metal mold is required. However, in this case, the metal mold can not be installed unless the forming machine is of a large size. For this reason, when the length of the shaft is 100 mm or more, there may be a case in which the existing forming machine can not be used for the processing so that a large-sized forming machine has to be introduced inevitably.
If the stopper portion and the shaft have the same diameter size, the forming machine requires a metal mold corresponding to the total length of the shaft, so that a large number of metal molds are required to be manufactured when a large kinds of products are to be produced on a small-lot basis. As a result, the unit cost of a small-lot products inevitably exceeds an allowed range if the production cost of such molds is included. Further, when a collapsible mechanism against a secondary collision by a driver is provided in a coupling element, as disclosed in the Japanese Patent Application Laid-Open No. 8-91230, the shaft is formed in the shape of a pipe so that the steering shaft can advance into the shaft. However, the processing therefor requires a large number of steps, thereby increasing the manufacturing cost of the shaft.
On the other hand, it is possible to employ a cold forging and pressing treatment which has less restrictions with respect to the size or the material of the shaft. However, in this case, an annealing step is required for softening a work which has been treated and hardened for each main forming step, so that the productivity drastically declines to inevitably increase the manufacturing cost, also.